In a firearm, the sear (traditionally defined as a sharp bar resting in a notch) is the part of the trigger mechanism that holds the hammer, striker or bolt until a correct amount of pressure is applied to the trigger, at which point the hammer, striker or bolt is released to discharge the cartridge from the weapon. As is further known, the sear may be a separate part or can be a surface incorporated into the trigger.
The sear on manual firearms is often connected to a disconnector forming a component of the fire control switch and which, after a single cartridge firing cycle in a semi-auto fire position, engages a configured location of the hammer to prevent the same from re-engaging the firing pin until such time as a succeeding and discrete trigger pull is executed.
A full auto sear supplements or replaces the traditional sear and operates to substitute for the disconnector by temporarily catching the hammer once it has been set by the rearwardly traveling bolt and following the hammer initially contacting the firing pin. The hammer includes a further configured notch location which is caught by a pivotally offset location of a “trip” of the sear (and again rather than the hammer being engaged by the disconnector).
Upon the bolt carrier traveling forward in its return buffer spring actuated direction, a notch on the underside of the carrier contacts an upper most location of the trip at a further pivotally offset location from where it is restraining the hammer, at which the continuing forward movement of the carrier causes the trip to be rotated a sufficient degree to release the hammer to again strike the firing pin, the timing for which permits the forward projecting bolt and extractor to strip a cartridge from the magazine and previously load the same into the chamber with the bolt lugs then pre-seated with the bolt receiver lugs.
An auto fire sear can include both those designed into the original manufacture of the firearm, as well as drop in auto sears which typically seat within the open rear interior of the lower receiver behind the trigger housing and which, in combination with a suitable configured fire control group (3 position firing selector with retractable disconnector and correctly designed hammer with secondary trip engaging notch) and bolt carrier group with suitably located notch for engaging the trip in the return direction, provide for converting a semi-automatic for full on auto fire.
The non-patent commercial publication entitled “Bev Fitchett's Guns Magazine—AR15 to M16 Conversion, Dec. 27, 2017, discusses one known version of a drop-in auto sear which teaches a main drop-in body for seating within the open rear of the upper interior behind the trigger group, the body having a modified and irregular rectangular shape with a “U” shaped forward facing profile. A trip component is pivotally supported, via a crosswise extending pin seating through the trip and the main body, between a pair of forwardly extending ears defining the “U” shaped profile.
A forward end face of the trip includes both a lower angled edge adapted to engage the hammer notch during a return upward stroke and an upper angled edge which, upon being contacted by a forwardly displacing underside notch of the bolt carrier during its return stroke, pivots the lower angled edge out of contact with the hammer. In this manner, a fast reciprocating continuous motion of the hammer corresponding to full auto fire occurs until such time as the user releases the trigger, resulting in the disconnector once again re-engaging the hammer in a final returning motion.
Other examples of auto sear constructions include such as that disclosed in US 2014/0338523, to Daley, Jr., which teaches an automatic sear assembly for providing a large-bore rifle with full-automatic firing and/or burst firing capabilities is disclosed. An automatic sear is operatively configured with a sear lever which is provided with bidirectional articulation for selectively imparting torque on the automatic sear to cause tripping thereof. The assembly can be configured such that rotational deflection of the sear lever away from the automatic sear imparts no rotation thereto, whereas rotational deflection of the sear lever toward the automatic sear imparts rotation thereto. Thus, and in accordance with some embodiments, the disclosed sear assembly can be used in a rifle, for example, to utilize the force of a moving bolt carrier during a given firing cycle to initiate a subsequent firing cycle without need to release and once again operate the trigger of the rifle.
Geissele, U.S. Pat. No. 9,759,504, teaches a sear mechanism for a firearm including a trigger element having one or more contact surfaces on which one or more movable or pivotable components of the sear mechanism selectively contracts or slides. In some embodiments, the trigger element is made by producing an intermediate workpiece of the trigger element by a manufacturing process and electric discharge machining the intermediate workpiece of the trigger element to provide the contact surfaces.
Larson, US 2014/0075814, teaches a gas piston style firearm having a bolt carrier, an adjustable gas piston block located forward on the firearm and an over-the-barrel spring and guide rod arrangement, all of which is housed and contained in a top rail that runs the length of the firearm and that maintains the alignment of these firearm components. The firearm further includes components that provide full auto firing capability. These components include a specially designed auto bracket that cooperates with a modified bolt carrier and a modified upper receiver.
Finally, US 2013/0118343, to Hirt, teaches an actuation system which includes a bolt carrier assembly configured for use within a firearm, the bolt carrier assembly including a firing pin configured for striking a chambered round. A hammer assembly is configured for striking the firing pin of the bolt carrier assembly. An auto sear assembly is configured to actuate the hammer assembly during operation of the firearm. A linkage assembly is configured to couple the bolt carrier assembly and the auto sear assembly and effectuate the auto sear assembly actuating the hammer assembly.